Effects of LaScO3 doping on structure, dielectric, and piezoelectric properties of K0.5Na0.5NbO3 piezoceramics

Abstract

Lead-free (1 − x)K0.5Na0.5NbO3–xLaScO3 (abbreviated as KNN–LSO) ceramic with (x = 0–0.10) was prepared via the solid-state synthesis route. The changes in the microstructure, crystal structure, dielectric, piezoelectric, and ferroelectric properties were analyzed for all compositions. The Rietveld refinement of powder x-ray diffraction suggested the variation in the crystal structure from orthorhombic for pure KNN to a mixed phase of tetragonal and orthorhombic for x ≥ 0.03. Temperature-dependent dielectric measurements also confirmed the change in the crystal structure with the increase in the LSO content, and the tetragonal to cubic TT-C phase transition temperature was found to shift toward room temperature. Interestingly, the temperature of the orthorhombic to tetragonal (TO-T) phase transition remained almost invariant in the LSO-doped sample (x ≥ 0.02), and a frequency dispersion in TO-T, typical for a relaxor ferroelectric, was observed. The frequency variation of temperature of the dielectric maximum Tm (∼426 K at 10 kHz and ∼444 K at 1 MHz) for x = 0.05 sample was fitted using the Vogel–Fulcher relation. This was further corroborated by the appearance of a slim hysteresis in the polarization–electric field plot. The impedance studies suggested that non-Debye relaxation and conduction mechanisms were associated with the thermal activation of oxygen vacancy that was rationalized by similar values of activation energy obtained from Arrhenius fitting for all samples. Current–voltage (I–V) measurements illustrate the reduction in room temperature DC conductivity with the increase in LSO doping. The highest room temperature piezoelectric charge coefficient (d33) value of 130 pC/N was observed for the composition corresponding to x = 0.01.